1. Field of the Disclosure
Embodiments disclosed herein relate to processes where the reactors have a relatively short effective operating cycle length wherein the catalyst activity is limited by fouling and require frequent regeneration. Embodiments disclosed herein specifically relate to the production of C2 to C3 olefins via the catalytic cracking of feedstocks including C4 and heavier olefins.
2. Background
In the processing of hydrocarbons, there are numerous catalytic processes where the catalyst fouls in time periods such that frequent regeneration is necessary. One example is the catalytic cracking of feedstocks where, in typical fluid bed systems, the catalyst loses activity in a time frame measured in minutes to hours depending upon conditions. In order to re-establish the activity of the catalyst, the coke produced in the reaction, which reduces the activity of the catalyst, must be removed under controlled conditions so as to avoid excessive temperature that would then sinter the catalyst and thus render it inactive.
As the length of time for the operating cycle decreases due to the fouling of the catalyst, the operator has limited choices. One option is to employ a fluid bed or equivalent reactor where a portion of the catalyst is withdrawn continually and regenerated in a separate vessel. This type of system, however, is costly. A second option is to use a series of fixed bed reactors where a portion of the reactors are operating and another portion are regenerating. These reactor systems operate in a cyclic manner where as one reactor fouls, one recently regenerated is brought on line while the fouled reactor is simultaneously taken offline to regenerate spent catalyst. However, these systems typically require numerous pieces of equipment specifically for regeneration service and other equipment for providing the heat of reaction.
One specific example where this becomes an issue is in the cracking of moderate carbon number olefins to lower olefins, specifically ethylene and propylene. Catalytic cracking is routinely used to convert heavier petroleum fractions such as gas oils and residual fractions to lighter products, and fluidized catalytic cracking (“FCC”) is particularly advantageous for heavy feeds. FCC is typically limited to feedstocks that produce sufficient coke on the catalyst, which when burned, generates sufficient heat to provide the necessary heat of reaction. The heavy feed contacts an appropriate catalyst and is cracked to form lighter products. The light products are typically gasoline and diesel oils, but fluid bed systems have been used to produce light olefins from heavy feeds.
Production of light olefins, for example, may also be effected by the catalytic cracking of moderate carbon number (C4 to C9) olefins. For example, C4 to C9 hydrocarbons may be cracked to form ethylene and propylene, among other products. Under these conditions, however, the amount of coke produced from these lighter feedstocks, is insufficient to overcome the endothermic heat of reaction, and separate heaters are required. In these cases additional equipment is required.
A variety of processes exist for the cracking of moderate carbon number olefins to produce lighter olefins, such as ethylene and propylene. However, such processes generally utilize conventional systems for feed preparation, reaction, and regeneration. For example, reactor systems, such as disclosed in U.S. Pat. Nos. 7,087,155 and 6,307,117, generally include use of two fixed bed reactor systems, where one reactor is operating and one reactor is regenerating. In these cases, separate operating equipment is required for the reaction systems and the regeneration systems.
Many recent endeavors to improve the cracking processes have investigated improvements to the catalysts used. However, conventional processes for the cracking of olefins, regardless of catalyst, often result in significant swings in product composition and conversion due to changes in catalyst performance over an operating cycle. Additionally, convention systems for feed preparation, reaction, and regeneration generally include multiple gas-fired heaters used for i) regeneration of the cracking catalysts and ii) heating of the feed to the cracker. Further, feed preparation systems typically utilize fixed bed hydrogenation systems to remove more highly unsaturated compounds, such as dienes, prior to entering the reactor. These systems typically operate in liquid phase and the resultant feed requires vaporization in additional equipment prior to entering preheating the feed for the reactor.
Accordingly, there exists a need for processes for the production of light olefins that may reduce the necessary operating costs and capital costs (e.g., process equipment piece count).